1. Field of the Invention
The present invention relates to a focus adjustment technique used in various types of image pickup apparatuses.
2. Description of the Related Art
Recently, autofocusing devices for video cameras have been predominantly the TV-AF type in which focus adjustment is performed as follows. Image sharpness is detected as an AF evaluation value from an image signal obtained through photoelectric conversion of an object image by using an image pickup device, for example, and a focusing lens position is controlled so that the AF evaluation value is maximized.
The AF evaluation value used in the TV-AF type is generally given as a level of a high-frequency component in an image signal extracted by a band-pass filter allowing passage of a certain band. Stated another way, as shown in FIG. 7, when an ordinary object image is picked up, the AF evaluation value is increased as a focusing lens approaches an in-focus position, and it is maximized at the in-focus position.
As another AF technique, there is known an internal phase-difference detection technique used in many single-lens reflex cameras. With the internal phase-difference detection technique, a light beam having passed through the exit pupil of an imaging lens is divided into two parts, and two divided light beams are received by sets of focus detection sensors, respectively. A deviation between signals output depending on quantities of the received light beams, i.e., a relative positional deviation in the dividing direction of the light beam, is detected. Based on the detected deviation, a deviation of the imaging lens in the focusing direction is determined directly. In other words, by performing a data accumulation operation once with the aid of the focus detection sensors, the amount and the direction of a focal deviation can be obtained and a focus adjustment operation can be realized with high speed.
As another type of phase-difference detection technique, there is also known an external phase-difference detection technique in which a distance sensor is disposed independently of an imaging lens. With the external phase-difference detection technique, a light beam received from an object is divided into two parts, and two divided light beams are received by sets of focus detection sensors, respectively. A deviation between signals output depending on quantities of the received light beams, i.e., a relative positional deviation in the dividing direction of the light beam, is detected. Based on the detected deviation, an object distance is determined by triangular surveying. Other examples of the AF technique using an external distance sensor include the type measuring a propagation speed by using an ultrasonic sensor, and the type performing triangular surveying by using an infrared sensor, which is employed in many compact cameras.
Further, there is known an autofocusing technique which is a combination of the above-mentioned autofocusing techniques. In one example, after moving the focusing lens to the vicinity of an in-focus position according to the internal phase-difference detection technique, a mode is changed to the TV-AF technique for further approaching to the in-focus position. (See Japanese Patent Laid-Open No. 5-64056 (page 4 and FIG. 1)).
However, the above-described known techniques have the following disadvantages. Generally, the in-focus position obtained by the technique other than the TV-AF type differs from an in-focus position obtained by the TV-AF technique. Also, an image pickup area where focus detection is performed by the TV-AF technique is not always matched with an area on an image plane, which is detected by the phase-difference detection technique described above. Therefore, different in-focus positions are calculated for the same object in some cases by the TV-AF technique and the phase-difference detection technique. When the plural AF techniques provide different in-focus positions in such a case, a blur is caused if the focusing lens is improperly moved to the in-focus position obtained by the phase-difference detection technique in spite of that the focusing lens is held at the in-focus position obtained by the TV-AF technique. Another undesired operation may also occur, for example, hunting such that the focusing lens repeats reciprocal movements between the different in-focus positions.